As the demand for increased data rates continues, a change from third generation (3G) to fourth generation (4G), long term evolution (LTE), specifications have returned for full bandwidth operation. In contrast to 3G wideband code division multiple access (WCDMA), 4G long term evolution (LTE) requires nearly band edge to band edge operation in both transmit and receive bands. The need comes as a result of the channel bandwidths for LTE. The channel bandwidths can change dynamically and can be as small as 1.4 MHz as opposed to the fixed 5 MHz channel bandwidths of WCDMA. As the channels shrink in size the channels at the edges of the bands become increasingly more dependent on a flatter response across the entire band. The amount of energy in the channels as the band edges are approached is higher and thus, the loss becomes more significant as channel spacing is reduced. It is especially difficult to obtain a flat response across the entire band for those bands with narrow separation between transmit (TX) and receive (RX) bands. As a result of this difficulty due to the close proximity between TX and RX bands, a high degree of TX to RX isolation for LTE is required where there is a tendency to roll filter responses off early to meet requirements detailed in rejection and isolation specifications.
Transceiver chip manufacturers for LTE applications have set expectations for performance that are beyond the capability of conventional Lithium Tantalate (LiTa) based surface acoustic wave (SAW) duplexer filters. LTE bands have proven to be relatively difficult for manufacturers of conventional LiTa SAW filters to maintain desired specifications. In particular, meeting requirements that will satisfy customers at the carrier level for the LTE band 2 and the LTE band 8 have proven to be extremely difficult. As a result of this problem, the third generation partnership project (3GPP) specifications have made some reluctant relaxations at the band edges. In efforts to maintain performance over the full band, some manufacturers have taken steps towards the use of more exotic technologies such as bulk acoustical wave (BAW) technology for LTE band 2. However, BAW technology has proven to be an expensive alternative to conventional SAW technology. For example, BAW filters are difficult to produce within manufacturing tolerances. As a result, lower yields of acceptable BAW filters are produced, which leads to higher costs. Attempts to meet LTE band 8 requirements have thus far lead to using bonded substrate technology, and various temperature compensating techniques. Such attempts and techniques have proven to be expensive and difficult to implement. What is needed is an architecture by which these types of very difficult bands, specifically LTE bands 2 and 8, are able to revert to using conventional LiTa SAW technology.